4 layers118 × 76 mm
Smart led badge
zgeez/Pixel-Badge
Wearable 32x32 LED badge with Zephyr firmware, BLE control, passive NFC, and a Flutter Android app, built around a custom nRF52840 PCB.
1,236 partsMIT
2 layers47.7 × 80.7 mm
Christmas tree ear ring
arvindsa/xmas-tree-earringA pair of wireless, synchronized LED earrings with nRF52 MCUs running deterministic animation patterns.
- Size
- 47.7 × 80.7 mm
- Copper layers
- 2
- Thickness
- 1.6 mm
- Parts
- 65
- Nets
- 67
- Pads
- 231
- Tracks, vias
- 716, 91
- Pours
- 2
- Routed
- Yes
- KiCad
- KiCad 9.0 (file 20241229)
- Created
- 31 May 2026
- Updated
- 31 May 2026
Overview
Christmas tree ear ring is an open-source printed circuit board by arvindsa, from the arvindsa/xmas-tree-earring repository on GitHub. It is a 2-layer board of 47.7 × 80.7 mm with 65 parts on 67 nets, routed with 716 tracks and 91 vias.
It is built around the AP2112K-3.3, MCP73831 and WS2812B-2020, with the 7M-16.000MAAJ-T, CM7V-T1A-32.768KHZ-9PF-20PPM-TA-QC and DMP2045U among its other parts. Its parts include 24 capacitors, 13 diodes, 8 connectors, 6 resistors and 3 ICs. It belongs with the nRF52 and nRF5x boards of this library.
Main components
7
AP2112K-3.3
SOT253.3V 600mA CMOS Low Dropout Linear Regulator with EnableDiodes IncorporatedUncategorised
MCP73831
UncategorisedMicrochip Technology
WS2812B-2020
SMD,2.2x2mm0.6mA 0.84mm 120us 160mcd 200mcd 2kHz 2mm 2mm 300ns 470.4nm 515.6nm 520mcd 5V 622.5nm 800Kbit/s Built-in power-on reset circuit、Built-in power-on reset circuit、Integrated intelligent reverse connection protection function Top-mount SMD,2.2x2mm RGB LEDs(Built-in IC) ROHSWorldsemiRGB LEDs(Built-In IC)
7M-16.000MAAJ-T
SMD3225-4P-20℃~+70℃ 16MHz 18pF 80Ω ±30ppm ±30ppm SMD3225-4P Crystals ROHSTXC CorpCrystals
CM7V-T1A-32.768KHZ-9PF-20PPM-TA-QC
SMD3215-2P-40℃~+85℃ 32.768kHz 70kΩ 9pF ±20ppm SMD3215-2P Crystals ROHSMicro CrystalCrystals
DMP2045U
SOT-23-55℃~+150℃ 1 P-Channel 1.7W 190pF 1V 20V 4.1A 7.8nC@4.5V 740pF 75mΩ@2.5V P-Channel SOT-23 MOSFETs ROHSTECH PUBLICMOSFETs
MBR0540
SOD-123-55℃~+150℃ 1 Independent 20uA@40V 40V 5.5A 500mA 510mV@500mA SOD-123 Schottky Diodes ROHSonsemiSchottky Diodes
README
README.mdXmas Tree Earring
A pair of wireless, synchronized LED earrings with nRF52 MCUs running deterministic animation patterns.
Hardware
- PCB Design: KiCad project files in
/pcb - Components: Custom PCBs with nRF52832 (master) and nRF52840 (slave), 12 WS2812B RGB LEDs each
- Power: Coin cell battery per earring
Firmware
- Master: nRF52832-based control unit with button for pattern selection
- Slave: nRF52840-based synchronized unit that mirrors the master's animation
Both units communicate via Nordic ESB (Enhanced ShockBurst) radio protocol to keep animations frame-perfect synchronized.
Building
Firmware builds require the nRF5 SDK and ARM GCC toolchain. See /firmware/README.md for details.
Design Documentation
- Sync mechanism and architecture: See firmware
ARCHITECTURE.mdfiles - Development log with debugging notes and design decisions in root
DEVLOG.md
Parts
65| Qty | Value | Footprint | References |
|---|---|---|---|
| 12 | WS2812B-2020 | LED_SMD:LED_WS2812B-2020_PLCC4_2.0x2.0mm | D1, D2, D4, D5, D6, D7, D8, D9, D10, D11, D12, D13 |
| 11 | 1u | Capacitor_SMD:C_0603_1608Metric | C7, C11, C12, C13, C14, C15, C16, C27, C28, C40, C41 |
| 6 | Conn_01x04_Pin | Connector_PinHeader_1.27mm:PinHeader_1x03_P1.27mm_Vertical | J3, J4, J5, J6, J7, J8 |
| 3 | 0.8p | Capacitor_SMD:C_0603_1608Metric | C8, C9, C10 |
| 3 | 100n | Capacitor_SMD:C_0603_1608Metric | C1, C2, C4 |
| 2 | 10K | Resistor_SMD:R_0603_1608Metric | R16, R21 |
| 2 | 12p | Capacitor_SMD:C_0603_1608Metric | C38, C39 |
| 2 | 4.7uF | Capacitor_SMD:C_0603_1608Metric | C23, C24 |
| 2 | Conn_01x02_Socket | Connector_PinSocket_2.54mm:PinSocket_1x02_P2.54mm_Vertical | J1, J2 |
| 2 | TestPoint | Arvind_TestPoint:Multicomp TEST-12 Test Point | TP1, TP2 |
| 1 | 100k | Resistor_SMD:R_0603_1608Metric | R10 |
| 1 | 100p | Capacitor_SMD:C_0603_1608Metric | C6 |
| 1 | 1M | Resistor_SMD:R_0603_1608Metric | R8 |
| 1 | 200 | Resistor_SMD:R_0603_1608Metric | R1 |
| 1 | 2K | Resistor_SMD:R_0603_1608Metric | R9 |
| 1 | 3.9nH | Inductor_SMD:L_0603_1608Metric | L1 |
| 1 | 4.7u | Capacitor_SMD:C_0603_1608Metric | C3 |
| 1 | 5.6nH | Inductor_SMD:L_0603_1608Metric | L2 |
| 1 | 7M-16.000MAAJ-T | Crystal:Crystal_SMD_2016-4Pin_2.0x1.6mm | Y2 |
| 1 | AP2112K-3.3 | Package_TO_SOT_SMD:SOT-23-5 | U5 |
| 1 | Antenna | RF_Antenna:Johanson_2450AT18x100_2400-2500Mhz | AE1 |
| 1 | CM7V-T1A-32.768kHz-9pF-20PPM-TA-QC | Crystal:Crystal_SMD_3215-2Pin_3.2x1.5mm | Y3 |
| 1 | DMP2045U | Package_TO_SOT_SMD:SOT-23 | Q1 |
| 1 | MBR0540 | Diode_SMD:D_SOD-123 | D3 |
| 1 | MCP73831 | Package_TO_SOT_SMD:SOT-23-5 | U6 |
| 1 | MountingHole | MountingHole:MountingHole_2.1mm | H1 |
| 1 | NC | Capacitor_SMD:C_0603_1608Metric | C5 |
| 1 | SW_Push | Arvind_Switch:SKSLLAE010 | SW_RST1 |
| 1 | SW_Wuerth_450301014042 | Arvind_Switch:SW_SPDT_PCM12 - Overhang | SW1 |
| 1 | nRF52832-QFxx | Package_DFN_QFN:QFN-48-1EP_6x6mm_P0.4mm_EP4.6x4.6mm | U1 |
Questions
How big is the Christmas tree ear ring PCB?
It measures 47.7 × 80.7 mm and has 2 copper layers, 1.6 mm thick.
What parts does the Christmas tree ear ring use?
65 parts, built around the AP2112K-3.3, MCP73831, WS2812B-2020 and 7M-16.000MAAJ-T. The full list of its parts is on this page, with links to each part's symbol and footprint where PCBKit's library has them.
Where can I download the Christmas tree ear ring KiCad files?
From the arvindsa/xmas-tree-earring repository on GitHub, at the commit this page was made from: the board is pcb/christmas-tree-ear-ring.kicad_pcb and its schematic pcb/christmas-tree-ear-ring.kicad_sch. PCBKit opens the board in the browser, with nothing to install.
Can I use the Christmas tree ear ring design in my own project?
Its repository states no licence, so its author keeps all rights: ask arvindsa before reusing the design.
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